The present invention relates to the homopolymerization of ethylene or the copolymerization of ethylene with an .alpha.-olefin at a high temperature of at least 180.degree. C. or more under a pressure exceeding 300 kg/cm.sup.2.
Recently, polymerization of ethylene has been carried out in the presence of a Ziegler type catalyst at high temperatures and pressures by using a polymerizer for high-pressure polyethylene, as is described in British Patent Specification No. 828,828.
It is also known that ethylene can be copolymerized with an .alpha.-olefin by this method so as to produce straight-chain, low-density polyethylene having a controlled density.
The above-mentioned method is very advantageous in that, in the industrial production of straight-chain, low-density polyethylene, an existing apparatus for producing high-pressure polyethylene can be used as it is, which eliminates the necessity of an additional investment of capital. However, the use of a Ziegler type catalyst at high temperatures and pressures involves the following drawbacks as compared with conventional low-temperature, low-pressure polymerization processes using the Ziegler type catalyst.
1. The catalyst gets rapidly deactivated at high temperatures and pressures, and the catalytic activity thereof is low. Even when the catalytic activity is high, if the catalyst residue contained in the resultant polymer is thermally unstable, the residue will be converted into a coloring matter which deteriorates the quality and/or hue of the polymer.
2. In the copolymerization of ethylene with an .alpha.-olefin at high temperatures and pressures, the .alpha.-olefin must be present in the polymerization vessel in a much higher concentration than desired for the resultant copolymer because the ethylene is very highly reactive, while the .alpha.-olefin has remarkably low reactivity. For this reason, the amount of the .alpha.-olefin used per unit product becomes remarkably large in the industrial production of straight-chain, low-density polyethylene, whereby the cost of production is increased in conjunction with a high price of the .alpha.-olefin.
Therefore, the polymerization of ethylene at high temperatures and pressures requires a catalyst having characteristics such that (i) it is highly active and capable of providing a polymer having an excellent hue and (ii) it is capable of providing a high degree of copolymerization of an .alpha.-olefin in the case of the copolymerization of ethylene with the .alpha.-olefin.